1. Field of the Invention
The present invention relates generally to a method, apparatus, and system for the management and control of flow streams. The present invention is a method, apparatus, and system for transferring a portion of a species of a first stream to a second stream, and more specifically to sealing systems and enhancement techniques that can improve the efficiencies of such a species transfer device.
2. Description of Related Art
Conventional heat transfer devices designed to remove both sensible and latent heat are commonly referred to as xe2x80x9centhalpy exchangers.xe2x80x9d For example, rotary air-to-air heat exchangers have the ability to transfer heat, water vapor, or both between incoming and outgoing ventilation airstreams. Exchangers of the rotary wheel type typically incorporate a media of heat exchange material (capable of absorbing sensible heat) coated with a desiccant material (capable of adsorbing moisture, and, thus latent as well as sensible heat). The incoming airstream is directed through one sector of the slowly revolving wheel while an outgoing airstream is directed through another sector of the wheel. Passages formed in the heat exchange media permit passage of the streams through the wheel.
Use of these devices in ventilating, heating and/or air conditioning systems can reduce heating and cooling costs, while providing fresh outdoor air to lower internal air pollution levels. Heat and moisture are simultaneously absorbed from the warmer airstream at the one sector and removed from the wheel by the cooler, dryer airstream at the other sector. For stationary periodic flow regenerators, the airstream is alternately directed through the entire exchange device first in one direction and then in the opposite direction.
In a broader sense, a stream conditioning device like an enthalpy exchanger is a management device of species potential between streams flowing through the exchanger, wherein the species are, for example, latent and sensible heat. Species transfer devices that can be used particularly well in connection with a fuel cell engine are disclosed in U.S. Pat. No. 6,013,385 and U.S. application Ser. No. 09/447,764, both herein fully incorporated by reference. Such species transfer devices can be positioned in the path of at least one reactant stream of a fuel cell""s inlet or outlet, which transfer devices condition that stream to improve the efficiency of the fuel cell.
In most fuel cell applications, relatively cool atmospheric air is used as the cathode reactant. The air must be heated and humidified before it is injected into the stack, or the membranes will dehydrate and the reaction will stop. In order to maintain proper operating conditions for the fuel cell, the temperatures and humidities of the anode and cathode circuits must be precisely controlled to avoid drying out the electrolyte or otherwise damaging the fuel cell, and thereby stopping the flow of electricity from the fuel cell. A sensible and latent heat transfer device such as an enthalpy wheel can be used for conditioning the pressurized oxidant. The enthalpy wheel operates by removing both sensible and latent heat from the fuel cell cathode exhaust stream to heat and humidify the cathode inlet stream.
The conventional enthalpy transfer device, such as a rotating heat exchanger for HVAC systems and the like, comprises a housing assembly incorporating a first and second inlet, and a first and second outlet, the housing assembly containing a rotatable heat wheel of exchange media with a desiccant. A heat-saturated first stream is routed through the first inlet and to an active section of the heat wheel, where sensible and latent heat are collected by the enthalpy wheel as the first stream passes therethrough via channels of the exchange media. Upon passing through the wheel, the first stream exits the assembly at the first outlet. As the enthalpy wheel rotates, this sensible and latent heat is released into the second stream, which second stream passes through the channels of the exchange media that were just traversed by the first stream. The rotation of the wheel causes each channel of the media to be exposed to the first stream, and then through a portion of heat wheel rotation, the second stream, alternately. The second stream thus accepts the heat of the first stream absorbed by the media and adsorbed by a desiccant of the heat wheel, and exits the housing via the first outlet. By controlling the rate of rotation of the enthalpy wheel and the rate of heat transfer, the relative humidity and wet bulb temperature of the streams can be controlled.
Through continued research and testing, it has been found that species transfer devices like those previously described can be improved upon in several respects. Namely, species transfer efficiencies can be optimized with certain modifications to the conventional transfer device. A first improvement is the provision of superior sealing systems limiting stream mixing during species transfer. The sealing system would prevent the higher-pressure stream from escaping into the lower-pressure stream. Preferably, the two streams do not mix at all in the housing. Yet another improvement includes the use of enhancement techniques to destabilize the thermal boundary level developed in the media, thus enhancing heat transfer. Further advantages are gained by varying the geometry of the media wheel, the types of media material used and the speed of rotation of the wheel, among many others. Another improvement includes formulating the heat wheel to resist changes in dimensions in response to changes in temperature. Thus it can be seen that a need yet exists for a lightweight and efficient means of species transfer that can be used in a number of different environments. It is to the provision of such a species transfer device meeting these and other needs that the present invention is primarily directed.
Briefly described, in a preferred form, the present invention is a species transfer method, device, and system capable of managing species potential in a flow. In one aspect of the present invention, a stream conditioning system for a fuel cell gas management system is disclosed. The stream conditioning system manages species potential in fuel cell reactant streams. In another aspect, the species transfer device can comprise a media element, an optional sealing system and an optional enhancement system.
The media element is a rotatable exchange media element enclosed within a housing assembly. In one aspect of the invention, the media element preferably does not expand or contract significantly in response to changes in temperature. In another aspect of the present invention, the media element comprises an exchange medium, preferably a ceramic medium, more preferably a magnesium aluminum silicate having an average linear coefficient of thermal expansion (CTE) at 25 to 800xc2x0 C. of less than about 20xc3x9710xe2x88x927/xc2x0 C., preferably less than about 10xc3x9710xe2x88x927/xc2x0 C., most preferably less than about 5xc3x9710xe2x88x927/xc2x0 C. In another aspect of the invention, the exchange medium has a CTE from about 1xc3x9710xe2x88x927/xc2x0 C. to about 20xc3x9710xe2x88x927/xc2x0 C., preferably about 2xc3x9710xe2x88x927/xc2x0 C. to about 10xc3x9710xe2x88x927/xc2x0 C., most preferred from about 3xc3x9710xe2x88x927/xc2x0 C. to about 8xc3x9710xe2x88x927/xc2x0 C. Having a CTE at 25 to 800xc2x0 C. of less than about 20xc3x9710xe2x88x927/xc2x0 C. enables the exchange medium to preserve the integrity of seals separating different streams, or to avoid seals entirely, thereby preventing separate streams from intermixing or leaking. Exemplary transfer media include but are not limited to ceramics and ceramic composites including but not limited to magnesium aluminum silicates. Suitable transfer media include natural or synthetic cordierite, cordierite containing substances, modifed cordierites, or semi-cordierites. The media element can be a unitary cylinder of exchange media, or can comprise a plurality of cylindrically axial wedges insertable into a spoke carriage designed to secure such wedges.
The exchange medium can be, but need not be, in the form of a wheel, and can be composed of a substance having a low CTE and at least partial sorbent characteristics. Thus, the exchange medium can but need not have an added sorbent. A sorbent is herein defined as a substance that has the ability to take up and hold species, as by absorption or adsorption. The sorbent can be chosen for its particular characteristics vis-à-vis the species to be transferred. If the species transfer device is used to transfer latent heat between streams, the species transfer device can be comprised of a substance that transfers latent heat, or the species transfer device can have an added sorbent such as a desiccant. If the species is CO, then the sorbent is a substance that can collect and release CO between the streams. In other aspects, the exchange medium itself can be a sorbent.
The optional sealing system enables the species transfer device to run efficiently through a range of pressures and velocities of flows by limiting the amount, if any, of flow mixture within the device. The sealing system can include dynamic seals of either the contact or clearance type, including wagon wheel end caps, a mattress assembly or a labyrinth seal, among others, to provide a sealing relationship between the rotating ends of the exchange media wheel and the housing assembly. The labyrinth seal can be used in conjunction with a sealing fluid that does not saturate the exchange media and is injected into leak paths in the exchange media.
It will be appreciated by those of ordinary skill in the art, that the present invention encompasses formulating the exchange medium including preferably, the heat wheel, to resist changes in dimensions in response to changes in temperature. Therefore, in one aspect of the present invention, the heat wheel is formulated to have a CTE low enough to prevent the wheel from changing shape and thereby preventing inlet and exhaust streams from commingling. The species transfer device having such exchange medium optionally includes a sealing system.
The present invention can also include an enhancement system. The enhancement system is capable of reducing the flow boundary layer build-up in the media element. The enhancement system is a heat transfer augmentation methodology, and in the disclosed embodiments, the magnitude of heat transfer enhancements exceeds the additional pressure drop and any electrical power consumption penalties. Three distinct enhancement systems include electrohydrodynamic (EHD) enhancement, fluidic oscillation and media shuffling, although more than one type of enhancement system can be utilized in one transfer device.
The transfer device can be designed for single-pass or multiple-pass flow paths. A single-pass device accepts a flow from one side of the unit, and exhausts that flow on the other side of the unit, wherein the flow makes a single-pass through the exchange media. A multiple-pass device, for example a double-pass device, enables the flow to traverse the wheel twice before being exhausted. In this double-pass embodiment, the length of the wheel can be half that used in relation to the length of the wheel in the single-pass device, but the wheel would be larger in diameter than the single-pass wheel. In an alternative embodiment of the double-pass device, the flow may traverse the length of the housing assembly twice, but only encounter media in one trip (length) through the device. In this alternative double-pass embodiment, the length of the wheel can be equal to that used in relation to the length of the wheel in the single-pass device, but the wheel would be smaller in diameter than the double-pass wheel that has media in both passes.
The transfer device can further incorporate a pressure equalizing system to reduce the pressure differences between active sections of the exchange medium, wherein active sections of the exchange medium are those sections of the exchange medium in flow communication with a stream.
Yet another improvement provided by the present invention is the option of a disposable media element with or without the sealing system, being thus a single replaceable unit of the transfer device.
Accordingly, it is an object of the present invention to provide a species transfer device for conditioning a flow, which device is compact, lightweight and inexpensive.
It is another object of the present invention to provide a species transfer device for conditioning a second flow with species from a first stream.
Another object of the present invention is to provide a species transfer device that utilizes a revolving sealing system and a wheel of unitary construction.
An object of the present invention is the utilization of enhancement techniques to reduce flow boundary layer build-up in the media element.
Yet another object of the present invention is to provide a species transfer device that utilizes a revolving sealing system and a wheel being formed from a plurality of wedge sections.
It is another object of the present invention to provide a species transfer device having a mattress assembly to provide a sealing relationship between the rotating ends of the exchange media wheel and the housing assembly of the device.
Further, it is another object of the present invention to provide a species transfer device that includes a disposable media wheel unit.
Another object of the present invention is to provide a species transfer device that utilizes a labyrinth sealing system for high pressure environments.
It is yet another object of the present invention to provide a species transfer device including a pressure equalizing system.
Yet another object of the present invention is to provide a species transfer device that enables multiple-pass flow paths through the exchange media, wherein the passes may be co-and/or counter-flow.
These and other objects, features, and advantages of the present invention will become more apparent upon reading the following specification in conjunction with the accompanying drawing figures.